Estimate of the modulation potential as a function of time. Panel (a): Production rate, Q, of ²⁶Al at depths of 60 and 400 g cm⁻² over time, the same as in Fig. 3. Panel (b): Ratio of the nuclide content, R, at two depths. Panel (c): Heliospheric modulation potential, ϕ, estimated with R from panel (b) and the curve in Fig. 2, as it would be observed at the given moment of time (age).